One of the primary steps in the fabrication of modern semiconductor devices is the formation of a thin film on a semiconductor substrate by chemical reaction of gases. Such a deposition process is referred to as chemical vapor deposition (CVD). Conventional thermal CVD processes supply reactive gases to the substrate surface where heat-induced chemical reactions can take place to produce the desired film. High density plasma CVD processes promote the disassociation of the reactant gases by the application of radio frequency (RF) energy to the reaction zone proximate the substrate surface thereby creating a plasma of highly reactive ionic species. The high reactivity of the released ionic species reduces the energy required for a chemical reaction to take place, and thus lowers the required temperature for such CVD processes.
In one design of high density plasma chemical vapor deposition (HDP-CVD) chambers, the vacuum chamber is generally defined by a planar substrate support, acting as a cathode, along the bottom, a planar anode along the top, a relatively short sidewall extending upwardly from the bottom, and a dielectric dome connecting the sidewall with the top. Inductive coils are mounted about the dome and are connected to a supply radio frequency generator. The anode and the cathode are typically coupled to bias radio frequency generators. A series of equally spaced gas distributors, typically nozzles, are mounted to the sidewall and extend into the region above the edge of the substrate support surface. The gas nozzles are all coupled to a common manifold which provides the gas nozzles with process gases, including gases such as argon, oxygen, silane, TEOS (tetraethoxysilane), silicon tetrafluoride (SiF.sub.4), etc., the composition of the gases depending primarily on what type of material is to be formed on the substrate. The nozzle tips have exits, typically orifices, positioned in a circumferential pattern spaced apart above the circumferential periphery of the substrate support and through which the process gases flow.
The thickness of the deposited film is ideally, but in practice is never, perfectly uniform. Deposition uniformity is very sensitive to source configuration, gas flow changes, source radio frequency generator current, bias radio frequency generator currents, the nozzle height above the substrate support and the lateral position of the nozzle relative to the substrate support. Improvements in this deposition uniformity are hindered by several factors. For example, it is often preferable that the height of the nozzles above the substrate support surface be greater than it is. However, for practical reasons it is not feasible to position the nozzles through the dielectric dome. Also, adjusting the height of the nozzles above the substrate for each process condition is not practical unless the substrate is movable vertically. Furthermore, while increasing the distance between nozzle orifices and the substrate tends to improve the deposition uniformity, it adversely affects the gas efficiency, that is requires the use of more gas or more time. In addition, argon is commonly directed through the manifold and nozzles as part of the process gases, argon flow contributing to the effectiveness of sputtering rate and sputtering uniformity. However, the use of argon restricts the flexibility one has in varying the flow rate of the process gases through the nozzles.
Another factor affecting deposition is related to the cleanliness of the nozzle orifices. Some process gases, such as silane, can thermally disassociate and deposit silica on the inside of the nozzle orifices. In addition, some oxygen may diffuse back into the nozzle orifices and react with the process gases to create a deposit on the inside of the nozzle orifices. Attempts to "dry clean" the chamber (by keeping the chamber closed and injecting a cleaning gas, such as fluorine compounds, into the chamber) can create additional problems. For example, fluorine gas can partially react with deposited silica and create a porous material which expands and clogs up the orifices even worse.